[Qemu-devel] Status of multithreading in user mode

[Cecilia Gonzalez-Alvarez]

Hi all,

I'm planning to use Qemu as a functional simulation frontend for a 
multicore simulator.

Although later plans include full-system simulation, I'd like to start, 
for the sake of simplicity, with the individual simulation of 
applications in user mode.

I'm interested in targets x86_64 and AArch64, and my general question 
is: how is the status of multithreading in user mode for those targets?
Concretely, can we trust that running multiple threads on qemu-x86_64 
and qemu-aarch64 will be reliable most of the time?

Going through the mailing list I have the feeling that multithreading 
and user mode is a recurring issue.

I've done some experiments with the latest Qemu from the repository 
(2.5.50) and although user mode for both architectures is mostly working 
well, it fails with this particular example for qemu-x86_64:
http://users.elis.ugent.be/~cgonzlez/fft.c

Compiled with GCC 4.8.4:
    gcc -mno-sse4 -mno-sse4.1 -mno-sse4.2 -mno-sse4a -mno-avx -mno-avx2 
-c -o fft.o fft.c
    gcc fft.o -lm -static -pthread  -o fft
and executed with 32 threads:
    qemu-x86_64 fft -p32
sometimes, randomly, stalls on a sys_futex.

I don't know if this is a very concrete bug that I should report 
somewhere else; my main concern is to know if threading support in user 
mode is almost fully implemented and stable with just some random and 
unimportant bugs.

Best,

Cecilia